C# 8.0 nullable and generics [duplicate]

Question

I'm playing around a bit with the new C# 8 nullable reference types feature, and while refactoring my code I came upon this (simplified) method:

public T Get<T>(string key)
{
    var wrapper = cacheService.Get(key);
    return wrapper.HasValue ? Deserialize<T>(wrapper) : default;
}

Now, this gives a warning

Possible null reference return

which is logical, since default(T) will give null for all reference types. At first I thought I would change it to the following:

public T? Get<T>(string key)

But this cannot be done. It says I either have to add a generic constraint where T : class or where T : struct. But that is not an option, as it can be both (I can store an int or int? or an instance of FooBar or whatever in the cache). I also read about a supposed new generic constraint where class? but that did not seem to work.

The only simple solution I can think of is changing the return statement using a null forgiving operator:

return wrapper.HasValue ? Deserialize<T>(wrapper) : default!;

But that feels wrong, since it can definitely be null, so I'm basically lying to the compiler here..

How can I fix this? Am I missing something utterly obvious here?

Answer 1

You were very close. Just write your method like this:

[return: MaybeNull]
public T Get<T>(string key)
{
    var wrapper = cacheService.Get(key);
    return wrapper.HasValue ? Deserialize<T>(wrapper) : default!;
}

You have to use the default! to get rid of the warning. But you can tell the compiler with [return: MaybeNull] that it should check for null even if it's a non-nullable type.

In that case, the dev may get a warning (depends on flow analytics) if he uses your method and does not check for null.

For further info, see Microsoft documentation: Specify post-conditions: MaybeNull and NotNull